Orion, the Hunter

Plus the planets for February.

Winter brings cold weather, but also the glowing gems that decorate the winter sky. Looking up, we can see many constellations that have bright stars in them. These include Canis Major, Canis Minor, Taurus, Gemini, Auriga and the gem of them all, Orion, the Hunter. Orion is composed of four stars (Rigel, Saiph, Bellatrix, and Betelgeuse) that make a vertical rectangle. In the center are three stars (Alnitak, Alnilam and Mintaka) in a straight line running slightly upward as you look from left to right, forming the belt of Orion. All these stars are first or second magnitude.

Click the image for a larger map.

Orion dominates the February evening sky. With its bright stars and various nebulae, it is one of the most interesting constellations in the sky. It has a famous supernova remnant (the Crab Nebula-M1), one of the brightest nebulae in the sky (Orion Nebula-M43) and dark nebulae like the Horsehead Nebula (Barnard 33). Its many bright stars make it easy to find.

Orion's mythology is a little bit hazy, but according to the myth, Orion was a giant hunter who was the son of the sea-god Poseidon and Euryale, daughter of Minos, king of Crete. From Poseidon he inherited the ability to walk on water, by which he reached the island of Chios and he attacked Merope, daughter of the king. The king blinded Orion and drove him away. Orion wandered in darkness with his servant Cedalion until he reached Lemnos. Hephaestus, the smith-god, had a forge there and told Cedalion to guide Orion to the utmost eastern point, where Helios healed his eyes.

Later, Orion returned to Crete and hunted with Artemis and her mother, Leto. During the hunt, Orion threatened to kill every beast on Earth. Hearing this, Mother Earth sent a giant scorpion, which stung and killed Orion. The goddesses implored Zeus to put Orion among the constellations. Zeus complied and added the scorpion as well. But to keep the two combatants apart, he put them in opposite sides of the sky, never to be seen at the same time again. Orion is in our winter sky and Scorpius is in our summer sky.

The Milky Way goes right through Orion, putting the constellation right in the plane of our home barred-spiral galaxy. We share the same spiral arm with the stars of Orion. Spiral arms are actually pressure waves traveling around the galaxy. They compress the dust and gas that make up our galaxy, starting the process of star formation.

Orion has an amazing number of first- and second-magnitude stars — seven — whereas most constellations have at most one or two. What makes Orion have such a concentration of bright stars?

As they march around our galaxy, the spiral arms occasionally encounter a large cloud of gas and compress it. This causes many stars to form from the cloud, all in the same small area. This is what happened when an arm of the Milky Way ran into what we now call the Orion Molecular Cloud. This giant cloud, composed mostly of hydrogen gas, was compressed in stages as the shock wave that forms the arm passed through it. About 10 million years ago, the shock wave first struck the cloud above and right of what is now Orion's belt.

The shock wave then moved downward, compressing the gas cloud to form Orion's belt and its neighbors, between 2 million and 5 million years ago. It continued to move south and formed most of the Orion's sword around 2 million years ago. Finally, the shock wave hit the area that now forms the Trapezium, the heart of the Orion nebula. The Trapezium is a tight cluster of eight stars that are less than a million years old.

The stars created in this process range from the bright ones that make up Orion to faint ones scattered throughout the constellation. All these stars form the Orion OB Association. The "OB" refers to the classification of these bright stars, types O and B. In many ways, the Orion OB Association is like a giant open cluster. It is between 1,100 and 1,650 light-years away, virtually in our own back yard!

The youngest part of the Orion OB Association is south of the center star of Orion's Belt. If you look there with the naked eye, you will find a hazy patch of light. With a telescope, you can tell that it is a nebula, gas and dust left over from the formation of the Trapezium being illuminated by those stars to form the Orion Nebula. This is one of the brightest and most famous nebulae in the sky. While it seems to be isolated, we are actually seeing an illuminated part of the remains of the original Orion Molecular Cloud.

Among the very young stars around the Orion Nebula, astronomers have found thousands of discs of dust and gas that hide protostars in the process of forming. While the visible light from these protostars is trapped inside the dust and gas discs that still surround them, the infrared light gets out and astronomers can then see them with infrared telescopes. One day we may have a problem recognizing our Orion with all the new stars that will break out in southern Orion over the next million years!

The Planets for February

Mercury is still in the evening sky as the month begins. It will be visible only for a little over a week as it heads back toward the Sun. At the beginning of the month, Mercury sets just after 7 p.m. and has a disc that is 7.4 seconds-of-arc across and 47% illuminated. The Messenger of the Gods is 11 degrees above the west-southwestern horizon as it gets dark and will be lower each day. It starts the month in Aquarius and moves eastward into the northern panhandle of Capricornus, where it passes the Sun on Feb. 15. It continues across Capricornus and back into eastern Aquarius, where it ends the month. As Mercury enters Aquarius, it becomes visible in the morning sky along the east-southeastern horizon around Feb. 20. By the end of the month, Mercury is eight degrees above the horizon as it starts to get light, shining at magnitude 0.9.

Watch the Skies

(Times MST)

Feb. 6, 12:22 p.m. — First Quarter Moon

Feb. 14, 4:53 p.m. — Full Moon

Feb. 15, 2 a.m. — Venus at greatest brilliancy

1 p.m. — Mercury passes between Earth and Sun (inferior conjunction)

Feb. 22, 10:15 a.m. — Last Quarter Moon

Feb. 27, 4 p.m. — Mercury stationary

Jupiter is 50 degrees up in the east as it gets dark. It is well placed for evening telescopic observation. You can see its 44.1-seconds-of-arc across disc as it climbs high in the evening sky. The King of the Gods is moving slowly westward in western Gemini and sets by 4:30 a.m. Jupiter shines at magnitude -2.6.

Mars is moving eastward in eastern Virgo, but it is slowing down and will come to a stop on the first day of March. It is slowly coming closer to the Earth, making its disc grow larger, 10.2 seconds-of-arc across at midmonth. The God of War rises in the east around 10:30 p.m., shining at magnitude -0.1.

The Ringed Planet comes up two hours later in the east-southeast. Glowing at magnitude +0.5, Saturn is moving slowly eastward in central Libra. At midmonth, its disc is 16.9 seconds-of-arc across with the Rings 38.4 seconds-of-arc across with the northern face showing. They are tilted down 22.6 degrees.

Venus has entered the morning sky for a nine-month run. At midmonth, it is magnitude -4.6 with a disc 40.3 seconds-of-arc across and is a 26% illuminated crescent. Venus rises at 4:15 a.m. on the east-southeastern horizon. Last month, Venus had just slipped over the Sagittarius-Scutum border and become stationary. This month it starts moving eastward, slipping back over the border into Sagittarius and ending the month in the eastern part of that constellation.

Leasburg Dam State Park just north of Las Cruces has a new public astronomical observatory. Established by the Las Cruces Astronomical Society and the State of New Mexico, it gives the public an opportunity to see the sky through a large telescope. Check with the park on the observing schedule and "keep watching the sky"!

An amateur astronomer for more than 40 years, Bert Stevens is co-director
of Desert Moon Observatory in Las Cruces.